The most common solution is to go to the hospital and talk to the alternative medicine guy. He'll give you $50 for the mushroom.

There are a few other ways also (possible spoilers):

- There is a girl at the beginning of the Peaceful Rest Valley who'll buy them off you.
- In the Saturn Valley using the hotspring will cure you there is also one in the Tenda Village. Also the doctor of the Saturn Valley will remove it for you for free.
- Any of the Song Spots will cure you and heal you completely.

Those are the ones off the top of my head.

Also Teddy Bears will absorb damage and can negate status effect PSI attacks. I don't know their hit points though.